Analog Devices AN578-a Application Notes

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Etienne Moulin

ADE7756 (i.e., AN-564 reference design) and, in addition

to the active energy measurement, provides the voltage

ADE7756 with a microcontroller. The communication

through a serial interface (SPI). The SPI port allows

the user to calibrate various components of the meter

to store various calibration parameters of the meter and

to store the meter’s data during a power-down.

tion of active energy. Circuitry is provided to null out

various system errors including gain, phase, and offset

All registers of the ADE7756 are available through the

to process rms calculation into the microcontroller. This

the meter.

14 of program memory, and a 4 kbytes

this document is to demonstrate that a cost-effective

voltage input.

further details on the accuracy achievements of this refer-

–2

–

THEORY OF CALCULATION

V t

RMS

= ×

( )

∫

the square root

V i

RMS

= ×

∑

( )

factor to the mean absolute value

RMS MAV

tive Power and the Apparent Power consumed by a load.

Active Power

Apparent Power

Active Power (t) Mean V(t) I(t)

Apparent Power V I

RMS RMS

= ×

( )

= ×

is the phase difference between the volt-

the reactive energy. When the active power and the real

Reactive Power Apparent Power Active Power

2 2

= −

Averaging

the more samples used for rms calculation, the more

the measurement depends on both measurement time

variation of these parameters in terms of dynamic range

the averaging time needed for an accurate rms measure-

Amplitude Dynamic Range:

Voltage 10:1

Period of Change of the Amplitude: 3 Seconds

Frequency of the Signal:

Windowing Effect

A commonly used method for canceling the windowing

–3

–

+ V

t) yields:

/2 + 2

cos (

t) + V

cos (2

DC COMPONENT (INCLUDING ERROR TERM) IS EXTRACTED BY THE LPF FOR REAL POWER CALCULATION

FREQUENCY – rad/s

0  2

2  V

 V

and 2

the rms calculation.

ADE7756 Sampling Mode

through a special mode called waveform sampling

ADE7756 data sheet). The sampling rate of the output

four ch o i c e s : CLKI N / 1 2 8 , CL K I N / 2 5 6 , CLKIN / 5 1 2 ,

IRQ

pin low for 16 µs

when a new sample is available in the waveform reg-

When the sampling period is over, another interrupt

0 0 0 01 HEX

SAMPLING RATE (CLKIN/128, CLKIN/256, CLKIN/512, OR CLKIN/1024)

IRQ

SCLK

DIN

DOUT

READ FROM WAVEFORM

CHANNEL 1 OR CHANNEL 2 DATA – 20 BITS

16s

SIGN

As discussed later, the rms computation implemented in

worst-case computation time of approximately 350 µs.

ADE7756 can be sped up to 5 MHz (see the ADE7756 data

ADE7756.

time needed for the microcontroller to process the data

through the required rms signal processing determines

the sampling frequency of the input.

With this frequency, the sampling periods available

through the ADE7756 are 35.7 µs, 71.5 µs, 143 µs, and

the signal processing, the lowest sampling rate is used

for the ADE7756 and the samples are read with a decima-

tion factor of 2. The actual sampling rate for the samples

which represents approximately 1.748 kHz.

SAMPLING RATE (CLKIN/1024)

IRQ

READING RATE = CLKIN/2048

–4

–

the rms calculation signal processing. Depending on the

tered to extract the dc component (see Figure 1). After the

tion and conversion to true rms information is then

very important because it reduces the computation time

Averaging/Low-Pass Filter

Averaging the output of the squared input can be done by

 ltering solution is implemented in the microcontroller

with a  rst order in nite impulse response  lter (IIR  l-

ter). The architecture of this  lter is shown in Figure 5; the

 lter reduces the number of mathematical operations

gain parameters and by limiting the

is done, the

–P

2N 2N+P

–

–1

1 (1+ 2 )

P 1

−

− −

where 2

is the operand of the IIR  lter.

2 2

SAMPLING

= ×

−

–P

is the actual operand of

the  lter.

VOLTAGE OR

CURRENT

INPUT

UPPER MSB

LPF

OFFSET GAIN

RMS

19 12

OR 17

–5

–

100 1000

1.000.100.01

–40

FREQUENCY – Hz

–20

–60

–70

H – dB

–10

–30

–50

–45dB ATTENUATION

@ 100Hz

–3dB CUTOFF FREQUENCY = 0.55Hz

frequency can be lowered but then it takes more samples

2500

4500 5500

1500500–500

SAMPLES

3500

ERROR 0.1%

1.2

1.0

0.8

0.6

0.4

0.2

0.0

At the given sampling frequency, 3,540 samples repre-

Errorn(%) =

( )

−

1 2

100

where

represents the number of samples.

When the output of the low-pass  lter has settled,

the square of the rms value of the input signal can be

value of the selected input on an LCD display.

Various algorithms exist to process the square root. A

the output and the output, depending on the result of the

value of the low-pass  lter is a 20-bit register; the square

SAR

–

An offset compensation is introduced to eliminate the

the LCD after a binary-to-decimal conversion.

voltage channel speci cs.

After the offset and gain compensation, the rms result

the digital signal processing to the format of the display.

–6

–

An interrupt servicing routine takes care of all the timers’

threshold (SAG detection). The microcontroller services

the interrupts (internal and external) as described in the

AN-564 reference design.

ADE7756. When an interrupt occurs, the microcontroller

tion (SAG pin = low) or a waveform sample available

ADE7756’s register. This functionality is very important to

tion mode. The power-down mode is described in more

RMS

CALCULATION MODE

ENABLE WFSM

AND SAG INTERRUPTS

REAL-TIME

RMS ROUTINE

DISABLE WFSM

INTERRUPT

POST-PROCESSING

RMS ROUTINE

NORMAL MODE

SAG PIN = HIGH

END OF

REAL-TIME RMS

ROUTINE?

INTERRUPT

YES

ENERGY DUMP

ROUTINE

INITIALIZATION

tronics energy meter.

the ADE7756 is used not only for energy accumulation,

to maintain the performance and behavior of the

AN-564 reference design.

When the ADE7756 is used in waveform sampling mode,

the active energy register cannot be read at the same

time. Nevertheless, the active energy is still accumulated

full-scale ac energy (see the ADE7756 data sheet). As the

NORMAL MODE

READ

ACTIVE ENERGY

STORE CONSUMED

ENERGY

IN EEPROM

CONFIGURATION OF

THE ADE7756

FOR RMS CALCULATION

RMS CALCULATION

READ

ACTIVE ENERGY

STORE CONSUMED

ENERGY IN EEPROM

CONFIGURATION OF

THE ADE7756

FOR NORMAL MODE

RMS

CALCULATION?

RMS VALUE

YES

–7

–

the rms information is not needed every three seconds,

the corner frequency of the low-pass  lter can be changed

to reduce the effect of the ripple frequency. The drawback

the rms information.

to deliver this information. There are no additional  rm-

ware routines necessary to process this information as

PercentageError

True RMS RMS Measurement

True RMS

100%=

− ×

tor, compared to 7 mA when the meter is in an energy

1000

1000.1

1.00%

VOLTAGE – V

MAX % ERROR

0.90%

0.80%

0.70%

0.60%

0.50%

0.40%

0.30%

0.20%

0.10%

0.00% 100

10.1